Recently, tomography apparatuses have been widely used in order to obtain information of the inside of a living organism (subject). There are an X-ray computed tomography (hereinafter “X-ray CT”) apparatus, a magnetic resonance imaging (MRI) apparatus, a single photon emission CT (hereinafter “SPECT”) apparatus, and a positron emission tomography (hereinafter “PET”) apparatus, as the tomography apparatuses. In the X-ray CT apparatus, X-ray beams having narrow widths are emitted to a certain cross section of the subject in multiple directions, X-rays permeating through the subject are detected, and a spatial distribution of the degree of abruption of the X-rays in the cross section is computed by a computer and imaged. Thus, dysplasia inside the subject such as a hemorrhagic area can be recognized by the X-ray CT.
On the other hand, since functional information in the subject can be obtained with high precision by the PET apparatus, development of the PET apparatuses has been progressing recently. In a diagnostic method using the PET apparatus, first, a medicine for inspection which is identified with a positron nuclide is introduced inside the subject by an injection, inhalation, or the like. The medicine for inspection introduced in the subject is stored in a specific portion having a function corresponding to the medicine for inspection. For example, in a case where a medicine for inspection of saccharide is used, the medicine is selectively stored in a portion where metabolism of a cancer cell or the like frequently occurs. At this time, a positron radiates from the positron nuclide of the medicine for inspection. At the time when the positron and an electron in the periphery of the positron are coupled and annihilated, two gamma rays (so-called annihilation gamma rays) are radiated in directions approximately 180 degrees relative to each other. These two gamma rays are simultaneously detected by a radiation detector provided surrounding the subject and an image is regenerated by a computer or the like, so that image data of the distribution of the radioisotopes (RI) of the subject are obtained. Thus, in the PET apparatus, since the functional information about the body of the subject is obtained, it is possible to elucidate the pathology of various intractable diseases.
As shown in FIG. 1, in a PET apparatus 100, gamma ray detectors 101 are provided so as to surround a subject S 360 degrees. The gamma ray detector 101 includes a semiconductor detector 102 and a detection circuit 103. Semiconductor detection devices (not shown in FIG. 1) are provided in the semiconductor detector 102. The detection circuit 103 is configured to electrically detect the gamma rays entering the semiconductor detection devices. In addition, a generating position of the gamma ray is identified based on an output signal indicating that the gamma ray has entered from the detection circuit 103 and position information of the semiconductor detection device indicating where the gamma ray has entered. Furthermore, by detecting multiple gamma rays, an image of the distribution of the medicine for inspection in the subject S is regenerated.
Since the annihilation gamma rays are radiated from the subject in random directions, multiple semiconductor detection devices are arranged in the semiconductor detector 102 so that detection efficiency is improved. For example, as shown in FIG. 2, a radiation detection unit 102 where boards 106 having semiconductor detection devices 105 are provided in a housing 104 has been suggested (see, for example, Patent Document 1).
[Patent Document 1] Japanese Laid-Open Patent Application Publication No. 2005-128000